1. Field of the Invention
The present invention relates to a method of forming a deposited film, and more particularly to a method of forming a deposited film by a microwave plasma CVD method in which raw material gas is decomposed by microwave energy to form the deposited film on a substrate. The method for forming the deposited film according to the present invention is preferably employed to form a thin film semiconductor such as a photovoltaic device, a thin film transistor (TFT), a sensor, and a photoreceptor for electrophotography.
The present invention relates to a photovoltaic device, and more particularly to a photovoltaic device that uses non-single-crystal silicon material.
The present invention relates to a method of continuously manufacturing photovoltaic devices such as solar cells, and more particularly to a method of mass-producing photovoltaic devices such as non-single-crystal silicon solar cells.
2. Related Background Art
Development and research of an application of a non-single-crystal silicone semiconductor material, for example, amorphous silicon (a-Si:H), to a photovoltaic device, which is a thin film semiconductor device, have been commenced in view of an invention of the photovoltaic device by D. E. Carlson (U.S. Pat. No. 4,064,521) based on the success in doping of a-Si:H by W. E. Spear and P. G. Le Comber (Solid State Communications, Vol. 17, pp. 1193 to 1196, 1975).
Recently, a photovoltaic device using a-Si:H has been put into practical use as a main power source or a subpower source for devices that use relatively small amounts of electric power such as clocks, small computers, and streetlights.
If a-Si:H deposited at a relative slow deposition speed (2 nm/sec or lower) by an RF plasma CVD method, which is a typical conventional film forming method, is used to form the i-layer of a photovoltaic device, the photovoltaic device has excellent characteristics. However, an increase in the deposition speed (2 nm/sec or higher) will cause excessive deterioration of the characteristics.
Therefore, in the case where a large amount of electric power is consumed, for example, in the case where an a-Si:H photovoltaic device is used for generating electricity, it must have more improved photoelectric conversion efficiency. Further, the light deterioration must be prevented. In order to further reduce the cost, technology is required that raises the deposition speed while maintaining the characteristics of the a-Si:H film and that is able to form a film over a large area with a high output.
A multiplicity of reports have been made about the deposition film forming method adapted to employ the microwave plasma CVD method.
For example, a microwave plasma CVD method making use of an ECR has been disclosed in "Chemical vapor deposition of a-SiGe:H films utilizing a microwave-excited plasma" T. Watanabe, M. Tanaka, K. Azuma, M. Nakatani, T. Sonobe, T. Simada, Japanese J. Appl. Phys., Vol. 26, No. 4, April 1987, pp. L288-L290; and "Microwave-excited plasma CVD of a-Si:H films utilizing a hydrogen plasma stream or by direct excitation of silane" T. Watanabe, M. Tanaka, K. Azuma, M. Nakatani, T. Sonobe, T. Simada, Japanese J. Appl. Phys., Vol. 26, No. 8, August, 1987, pp. 1215-218.
Japanese Patent Laid-Open No. 59-16328 "PLASMA VAPOR-PHASE REACTION APPARATUS" has disclosed a method of depositing a semiconductor film by a microwave plasma CVD method. Japanese Patent Publication No. 59-56724 "METHOD OF FORMING THIN FILM UTILIZING MICROWAVE PLASMA" has also disclosed a method of depositing a semiconductor film by a microwave plasma CVD method.
A method adapted to utilize the RF plasma CVD method and having an arrangement such that a mesh-like third electrode is formed between the anode and the cathode has been disclosed in "Preparation of highly photosensitive hydrogenated amorphous Si--Ge alloys using a triode plasma reactor" A. M. Atsuda, et al., Applied Physics Letters, Vol. 47, No. 10, 15 Nov., 1985, pp. 1061-1063.
In the conventional method of forming a deposited film adapted to the microwave plasma CVD method, insufficient deposition speed, unsatisfactory electric characteristics and excessive deterioration occur if it is used while being irradiated with light for a long time, as for example, when a non-single-crystal semiconductor film (an amorphous silicon a-Si:H, for example) is deposited. In particular, the foregoing problems of the electrical characteristics of the semiconductor film, contact with substrate, and the light deterioration are critical if the deposition speed is raised.
In the conventional method of forming a deposited film adapted to the microwave plasma CVD method, the plasma cannot easily be made uniform. That is, the thickness and the characteristics of the formed deposited film cannot easily be made uniform, and accordingly, the characteristics and the yield of the solar cell, the thin-film transistor, and the light receiving member for electrophotography deteriorate.
In the conventional method of forming a deposited film adapted to the microwave plasma CVD method, the plasma is unstable in a region in which the quantity of the raw material gas for forming the deposited film is insufficient.
In order to produce a photovoltaic device which meets the demand for electricity, the photovoltaic device must basically exhibit satisfactory photoelectric conversion efficiency and excellent stability of the characteristics. Furthermore, mass production of the photovoltaic devices must be enabled. In order to meet this, the manufactured photovoltaic device using the a-Si film or the like must have improved electric, optical, photoconductive, or mechanical characteristics and fatigue resistance or environmental resistance characteristics. Furthermore, the photovoltaic device must be enabled to be formed over a large area and must exhibit uniform film thickness and quality of the film. Furthermore, mass production must be enabled by forming the film at high speed while maintaining reproducibility.
The system for generating electricity using the photovoltaic device usually has an arrangement that unit modules are connected in series or in parallel to form an array so that a desired electric current or voltage level is obtained. Therefore, each module must be free from disconnection and short circuit. Further, the output voltage and output current must be uniform between modules. Therefore, the uniformity of the characteristics of the semiconductor layer, which is the most important factor determining the characteristics, must be maintained at least in the process for manufacturing the unit module. From the viewpoint of easily designing the module and simplifying the module assembling process, a semiconductor deposited film which can be formed over a large area and which exhibits excellent uniformity of the characteristics must be provided in order to improve the mass-production facility of the photovoltaic devices and to significantly reduce the manufacturing cost.
The photovoltaic device comprises the semiconductor layers, which are the important components thereof, the semiconductor layers being connected by a so-called pn junction or pin junction. In a case where the thin film semiconductor is made of a-Si for example, phosphine (PH.sub.3) or diborane (B.sub.2 H.sub.6) gas and serving as a dopant is mixed with silane serving as the main raw material gas to perform glow discharge decomposition.
As a result, semiconductor films having a desired conductivity type can be obtained, the semiconductor films being then sequentially stacked on a desired substrate. Therefore, the semiconductor junction can easily be realized. Accordingly, a method for manufacturing the a-Si type photovoltaic device has been suggested which has an arrangement such that independent film forming chambers are provided for the corresponding semiconductor films and the semiconductor layers are individually formed in the film forming chambers.
U.S. Pat. No. 4,400,409 has disclosed a continuous plasma CVD apparatus adapted to a roll-to-roll method. The apparatus comprises a plurality of glow discharge regions and has an arrangement such that a flexible substrate having a desired width and a sufficient length is disposed in a passage sequentially passing through the glow discharge regions. By continuously conveying the substrate in its longitudinal direction while forming a semiconductor layer having a desired conductivity type in the glow discharge region, the devices having the semiconductor junction can be manufactured continuously. In each of the deposition regions, dispersion and undesirable mixture of the dopant gas for use at the time of manufacturing the semiconductor layers into the other glow discharge regions are prevented by using a gas gate. Specifically, the glow discharge regions are separated from each other by slit-like separation passages. Furthermore, a means for creating a flow of scavenging gas, for example Ar or H.sub.2, is employed in the separation passage.
Although the roll-to-roll method is suitable to mass-produce the semiconductor devices as described above, the photovoltaic devices must be further improved in photoelectric conversion efficiency, the stability and uniformity of the characteristics and by reducing the manufacturing cost.
In order to improve the photoelectric conversion efficiency and stability of the characteristics, the photoelectric conversion efficiency and the deterioration ratio of the characteristics of each unit module must, of course, be improved by 0.1% units (corresponding to a ratio of about 1.01 times). Further, the unit module having the minimum current or the voltage characteristics in the unit module constituting the unit serves as a speed determining factor in the case where the unit modules are connected in series or in parallel to constitute an array. Therefore, it is very important to reduce scattering of the characteristics as well as improving the average characteristics of the unit module. Therefore, there is a need to maintain the uniformity of the characteristics of the semiconductor layer which are the most important character determining factors in this stage of manufacturing the unit module. In order to reduce the manufacturing cost, there is a need to improve the manufacturing yield by decreasing the number of defects occurring in the semiconductor layer for the purpose of preventing disconnection and short circuit.
Therefore, to attain this, the semiconductor layer must be deposited on an elongated member which is continuously moving, and by an improved film-forming method by which the uniformity of the characteristics can be realized and the defects can be decreased.